Tight Sandstone Reservoir Formation Mechanism of Upper Paleozoic Buried Hills in the Huanghua Depression, Bohai Bay Basin, Eastern China

Carboniferous-Permian petroleum resources in the Huanghua Depression of the Bohai Bay Basin, a super petroleum basin, are important exploration successor targets. The reservoir sedimentary environment of coal measures in the Upper Paleozoic buried hills is variable, and the structural evolution process is complicated, which restricts the optimization of targeting sections. Using the analysis and testing results of logging, thin section, porosity, mercury injection, hydrochemistry, and basin simulation, this study revealed the formation mechanism differences of tight sandstones in the Upper Paleozoic period in different buried hills. The results show that the sandstones are mainly feldspathic sandstone, lithic arkose, feldspathic lithic sandstone, and feldspathic lithic quartz sandstone. The quartz content varies between 25% and 70%, averaging 41%. Feldspar and debris are generally high, averaging 31% and 28%, respectively. Secondary dissolution pores are the main reservoir spaces, with 45% of the tested samples showing porosity of 5–10%, and 15% being lower than 5%. The pore radium is generally lower than 100 nm, and the sandstones are determined as small pore with fine throat and medium pore with fine throat sandstones by mercury saturation results. Frequent changing sedimentary environments and complex diagenetic transformation processes both contribute to the reservoir property differences. The former determines the original pore space, and the latter determines whether they can be used as effective reservoirs by controlling the diagenetic sequences. Combining tectonic movement background and different fluid history, the different formation mechanisms of high-porosity reservoirs are recognized, which are atmospheric leaching dominated (Koucun buried hills), atmospheric water and organic acid co-controlled (Wangguantun and Wumaying buried hills), and organic acid dominated (Nandagang buried hills) influences. The results can be beneficial for tight gas exploration and development in coal measures inside clastic buried hills in the Bohai Bay Basin.

Identification of Breakout Behind Casing: Methodology to Obtain Openhole-Equivalent Caliper Measurements Through Slotted Liner Using the Density Tool

Growth in the coal seam gas industry in Queensland, Australia, has been rapid over the past 15 years, with greater than USD 70 billion invested in three liquified natural gas export projects supplied by produced coal seam gas. Annual production is of the order of 40 Bscm or 1,500 PJ, with approximately 80% of this coming from the Jurassic Walloon Coal Measures of the Surat Basin and 20% from the Permian Coal Measures of the Bowen Basin. The Walloon Coal Measures are characterized by multiple thin coal seams making up approximately 10% of the total thickness of the unit. A typical well intersects 10 to 20 m of net coal over a 200- to 300-m interval, interbedded with lithic-rich sandstones, siltstones, and carbonaceous mudstones. The presence of such a significant section of lithic interburden within the primary production section has led to a somewhat unusual completion strategy. To maximize connection to the gas-bearing coals, uncemented slotted liners are used; however, this leaves fluid-sensitive interburden exposed to drilling, completion, and produced formation fluids over the life of a well. External swellable packers and blank joints are therefore used to isolate larger intervals of interburden and hence minimize fines production. Despite these efforts, significant fines production still occurs, which leads to the failure of artificial lift systems and the need for expensive workovers or lost wells. Fines production has major economic implications, with anecdotal reports suggesting up to 40% of progressive cavity pump artificial lift systems in Walloon Coal Measures producers may be down at any one time. The first step in solving this problem is to identify the extent and distribution of fines production. The wellbore completion strategy above, however, precludes the use of mechanical calipers to identify fines-production-related wellbore enlargement. A new caliper-behind-liner technique has therefore been developed using a multiple-detector density tool. Data from the shorter-spacing detectors are used to characterize the properties of the liner as well as the density of the annular material. This is particularly important to evaluate as the annulus fill varies between gas, formation water, drilling and completion fluids, and accumulated fines. The longer-spacing detector measurements are then used in conjunction with pre-existing openhole formation density measurements to determine the thickness of the annulus, and hence hole size, compensating for liner and annulus properties.

Basin Analysis of Tertiary Strata in Golden Bay, Nelson

<p>Eocene to Miocene strata comprise the Brunner Coal Measures (Ak-Ld), Takaka Limestone (Ld-Po), Tarakohe Mudstone (P1-S1) and Waitui Sandstone (Sc-Sl), and form a transgressive-regressive sequence on an essentially stable structural platform. Brunner Coal Measures in the Takaka valley (up to 350m thick) consist of cross-bedded sand and gravel, interlaminated sand and silt, bioturbated muddy sandstone, carbonaceous mudstone and coal. Five facies associations are recognized and interpreted as river/floodbasin, estuarine and shallow marine deposits. In the Aorere and Parapara River catchments, two new members are recognized: the Quartz Wash Member, comprising quartzose sand and conglomerate, and the Washbourn Limonite Member, a sedimentary iron-ore deposit. The Takaka Limestone (up to 100m thick), consists of bryozoan, bivalve or sandy grainstone or packstone, deposited on a tidal current-swept shallow-middle shelf with minimal terrigenous influx. Diagenesis was controlled by pressure-solution during deep burial, and resulted in a rightly cemented rock with dolomite and neomorphic features. The Tarakohe Mudstone (up to 900m thick) is dominated in its lower half by massive mudstone of hemipelagic and turbiditic origin, and in its upper half by shallow shelf-estuarine sandstones and mudstones. The Waitui Sandstone (160m thick) comprises shallow marine sandstone. Deposition of the Brunner Coal Measures took place in localized fault-angle depressions. The Takaka Limestone was deposited during a period of regional subsidence and minimal tectonic activity. The Tarakohe Mudstone and Waitui Sandstone were deposited in synclinal basins which were later modified by rising monoclinal boundaries.</p>

Basin Analysis of Tertiary Strata in Golden Bay, Nelson

<p>Eocene to Miocene strata comprise the Brunner Coal Measures (Ak-Ld), Takaka Limestone (Ld-Po), Tarakohe Mudstone (P1-S1) and Waitui Sandstone (Sc-Sl), and form a transgressive-regressive sequence on an essentially stable structural platform. Brunner Coal Measures in the Takaka valley (up to 350m thick) consist of cross-bedded sand and gravel, interlaminated sand and silt, bioturbated muddy sandstone, carbonaceous mudstone and coal. Five facies associations are recognized and interpreted as river/floodbasin, estuarine and shallow marine deposits. In the Aorere and Parapara River catchments, two new members are recognized: the Quartz Wash Member, comprising quartzose sand and conglomerate, and the Washbourn Limonite Member, a sedimentary iron-ore deposit. The Takaka Limestone (up to 100m thick), consists of bryozoan, bivalve or sandy grainstone or packstone, deposited on a tidal current-swept shallow-middle shelf with minimal terrigenous influx. Diagenesis was controlled by pressure-solution during deep burial, and resulted in a rightly cemented rock with dolomite and neomorphic features. The Tarakohe Mudstone (up to 900m thick) is dominated in its lower half by massive mudstone of hemipelagic and turbiditic origin, and in its upper half by shallow shelf-estuarine sandstones and mudstones. The Waitui Sandstone (160m thick) comprises shallow marine sandstone. Deposition of the Brunner Coal Measures took place in localized fault-angle depressions. The Takaka Limestone was deposited during a period of regional subsidence and minimal tectonic activity. The Tarakohe Mudstone and Waitui Sandstone were deposited in synclinal basins which were later modified by rising monoclinal boundaries.</p>

IDENTIFICATION OF BREAKOUT BEHIND CASING: METHODOLOGY TO OBTAIN OPENHOLE EQUIVALENT CALIPER MEASUREMENTS THROUGH SLOTTED LINER USING THE DENSITY TOOL

Growth in the coal seam gas industry in Queensland, Australia, has been rapid over the past fifteen years, with greater than USD 70 billion invested in three liquified natural gas export projects supplied by produced coal seam gas. Annual production is of the order of 40 Bscm or 1,500 PJ, with approximately 80% of this coming from the Jurassic Walloon Coal Measures of the Surat Basin and 20% from Permian coal measures of the Bowen Basin. The Walloon Coal Measures are characterized by multiple thin coal seams making up approximately 10% of the total thickness of the unit. A typical well intersects 10 to 20 m of net coal over a 200 to 300 m interval, interbedded with lithic-rich sandstones, siltstones, and carbonaceous mudstones. The presence of such a significant section of lithic interburden within the primary production section has led to a somewhat unusual completion strategy. To maximize connection to the gas-bearing coals, uncemented slotted liners are used; however, this leaves fluid-sensitive interburden exposed to drilling, completion, and produced formation fluids over the life of a well. External swellable packers and blank joints are therefore used to isolate larger intervals of interburden and hence minimize fines production. Despite these efforts, significant fines production still occurs, which leads to failure of artificial lift systems and the need for expensive workovers or lost wells. Fines production has major economic implications, with anecdotal reports suggesting up to 40% of progressive cavity pump artificial lift systems in Walloon Coal Measures producers may be down at any one time. The first step in solving this problem is to identify the extent and distribution of fines production. The wellbore completion strategy above, however, precludes use of mechanical calipers to identify fines production-related wellbore enlargement. A new caliper-behind-liner technique has therefore been developed using a multiple-detector density tool. Data from the shorter spacing detectors is used to characterize the properties of the liner as well as the density of the annular material. This is particularly important to evaluate as the annulus fill varies between gas, formation water, drilling and completion fluids, and accumulated fines. The longer spacing detector measurements are then used in conjunction with pre-existing open-hole formation density measurement to determine the thickness of the annulus, and hence hole size, compensating for liner and annulus properties. This methodology has been applied to several wells completed in the Walloon Coal Measures. Results have demonstrated the ability to identify zones of borehole enlargement behind slotted liner, as well as intervals of either gas or fines accumulation in the annulus. In addition, the technique has been successful in verifying the placement of swellable packers and their integrity. The application of this solution has been used to drive improvements in the design of in-wellbore completion programs and in the future will help drive recompletion decisions and trigger proactive workovers.

Optimization of Coal Seam Connectivity via Multi-Seam Pinpoint Fracturing Operations in the Walloons Coal Measures, Surat Basin

The Walloons coal measures located in Surat Basin (eastern Australia) is a well-known coal seam gas play that has been under production for several years. The well completion in this play is primarily driven by coal permeability which varies from 1 Darcy or more in regions with significant natural fractures to less than 1md in areas with underdeveloped cleat networks. For an economic development of the latter, fracturing treatment designs that effectively stimulate numerous and often thin coals seams, and enhance inter-seam connectivity, are a clear choice. Fracture stimulation of Surat basin coals however has its own challenges given their unique geologic and geomechanical features that include (a) low net to gross ratio of ~0.1 in nearly 300 m (984.3 ft) of gross interval, (b) on average 60 seams per well ranging from 0.4 m to 3 m in thickness, (c) non-gas bearing and reactive interburden, and (d) stress regimes that vary as a function of depth. To address these challenges, low rate, low viscosity, and high proppant concentration coiled tubing (CT) conveyed pinpoint stimulation methods were introduced basin-wide after successful technology pilots in 2015 (Pandey and Flottmann 2015). This novel stimulation technique led to noticeable improvements in the well performance, but also highlighted the areas that could be improved – especially stage spacing and standoff, perforation strategy, and number of stages, all aimed at maximizing coal coverage during well stimulation. This paper summarizes the findings from a 6-well multi-stage stimulation pilot aimed at studying fracture geometries to improve standoff efficiency and maximizing coal connectivity amongst various coal seams of Walloons coal package. In the design matrix that targeted shallow (300 to 600 m) gas-bearing coal seams, the stimulation treatments varied in volume, injection rate, proppant concentration, fluid type, perforation spacing, and standoff between adjacent stages. Treatment designs were simulated using a field-data calibrated, log-based stress model. After necessary adjustments in the field, the treatments were pumped down the CT at injection rates ranging from 12 to 16 bbl/min (0.032 to 0.042 m3/s). Post-stimulation modeling and history-matching using numerical simulators showed the dependence of fracture growth not only on pumping parameters, but also on depth. Shallower stages showed a strong propensity of limited growth which was corroborated by additional field measurements and previous work in the field (Kirk-Burnnand et al. 2015). These and other such observations led to revision of early guidelines on standoff and was considered a major step that now enabled a cost-effective inclusion of additional coal seams in the stimulation program. The learnings from the pilot study were implemented on development wells and can potentially also serve as a template for similar pinpoint completions worldwide.